MT- equals VERY OLD- earliest runs of MIKES TRAINHOUSE
Again, this is generally the first runs of many of these items in the late 1990s- say 1998.
This is when they were using QSI reverse boards, then the very first iterations of Protosound 1. Generally, they are nice and well built locomotives, the same basic mechanical designs MTH is still using today as far as motor drives, gearing, basic design elements.
The Catch22 is often- if PS1, the battery if never replaced is long shot, they can be very sensitive to what transformer is used (these really want pure sine wave).